Abstract No study has been performed to prospectively record the presenting complaints, signs, symptoms and comorbidities of children who raised the suspicion of pulmonary embolism (PE) in children. Clinicians must extrapolate what is known about PE in adults to children. This process puts children in harm?s way: most children with PE go undiagnosed after their first contact with a healthcare provider, meaning diagnosis is often delayed. Perhaps as a result, the mortality rate of PE is the same in children as it is in adults. On the other hand, tens of thousands of low risk children and adolescents are unnecessarily exposed to increased lifetime cancer risk from ionizing radiation from CT scans done to search for PE they do not have. No clinical criteria have been developed to either estimate the probability of PE in children, or to exclude PE based upon information available at the bedside. In adults, the Pulmonary Embolism Rule out Criteria, or PERC rule, has been validated as a method to exclude PE. In this work, we will test a modification of the PERC rule, PERC- Peds, which uses an age adjustment to heart rate, one of the 8 objective criteria in the PERC rule. This project will prospectively test if PERC-Peds, can safely exclude PE in 4,030 children aged 5-17 who raise a suspicion of PE in the emergency department setting. The criterion standard outcome is diagnosis of PE or DVT within 45 days. To reach the study-defined threshold of safety, the top limit of the 95% confidence interval for the false negative rate of the exclusionary criteria (PERC-Peds) must be lower than 1.5%. Additionally, we will test the diagnostic accuracy of the D-dimer, in the subset of patients with the test ordered as part of usual care, and data will be collected for multiple other ancillary aims. This project will also identify which factors truly increase or decrease the probability of PE outcome in children who raise a suspicion of PE, and may provide a much needed evidence basis for physicians to have a rational basis to launch a workup for PE, and also provide clinical criteria to exclude PE without the need for ionizing radiation.